1. Field of the Invention
The present invention relates to a heat transfer system adapted to evacuate dissipated power generated by equipment on board a spacecraft and in particular a satellite.
2. Description of the Prior Art
During operation of a spacecraft in orbit, onboard electrical and electronic equipment dissipates a quantity of heat which is dependent on the intrinsic efficiency of the equipment and can be high in some high-power equipment. To maintain the thermal environment of the equipment in temperature ranges compatible with its operation and performance, it is necessary to provide a heat transfer system to collect and transport the heat and then evacuate it into space.
A number of prior art systems perform this transfer of heat. They include a system for transporting and distributing heat and a system for transferring heat by radiation. The principle of the heat transfer system is based on the use of a fluid circulating between a hot area in which heat is dissipated and a colder area in which heat absorbed by the fluid is evacuated to an external medium by radiation via one or more fixed or deployable radiators. The operating principle of the system is therefore based on its transport and exchange capacity and, in the case of two-phase systems, the fluid evaporation/condensation properties (the latent heat of the fluid).
A first type of prior art system is known as a heat pipe. This system includes a rigid metal tube (for example an aluminum tube) in which a heat exchange fluid (generally ammonia) circulates and relies on liquid-vapor phase change properties and the capillarity properties of a liquid. Thus a heat pipe is a closed two-phase system in which vapor generated in the hot area (referred to as the evaporation area) is aspirated toward a colder area (where the pressure is lower) and condenses therein on the metal wall of the tube. The liquid phase of the fluid flows along the metal wall of the tube in the opposite direction to the flow of the vapor phase of the fluid, which remains confined to the center of the tube. The fluid is returned along the wall by a capillary structure (a wick or longitudinal grooves) linking the two ends of the tube and serving as a capillary pump and as a separator of the liquid-vapor phases.
Heat transfer systems based on heat pipes are frequently used in satellites, but have two major limitations. Firstly, the mechanical rigidity of heat pipes means that they cannot be used to transport heat to radiating surfaces that are deployable in orbit (deployable radiators), as this necessitates reconfiguring the heat path in space. Secondly, in the case of high-power telecommunication satellites, their limited performance in terms of heat transport capacity (which is the order of a few hundred W.m) and transport distance necessitates the use of different systems in which heat can be transported over greater distances, along more complicated heat paths, which are sometimes three-dimensions and reconfigurable in flight (to deploy radiators) and the transport function may need to be actively assisted by mechanical pumping. These heat transfer systems are fluid loop systems. The loops can be single-phase with pumping, two-phase with capillary pumping, or two-phase with mechanical pumping. Fluid loop systems have three parts: an evaporator, a radiator, and flexible or rigid fluid lines.
The operating principle of single-phase fluid loop systems is similar to that of central heating using the sensible heat of the fluid. The heat-exchange fluid (Freon™, water, ammonia, etc.) absorbs power dissipated by the equipment, thereby increasing in temperature in the vicinity of the heat source, and rejects that power when it cools in one or more radiators, without changing its physical state. The fluid is pumped by an active pumping system. Mechanical pumping is effected by an electrically powered pump providing the required flowrate of fluid in the loop. Quite apart from their energy consumption, mechanical pumps can generate microvibrations that can be incompatible with other equipment and instrumentation onboard the satellite. Moreover, they can also have a durability (service life) that represents a constraint on the mission of the satellite, in that their durability is limited by premature wear of some of the internal mechanical parts of the pump.
Like heat pipes, two-phase fluid loops use, in addition to the increase in temperature, the latent heat of evaporation of the fluid to absorb and reject heat. Thus the heat-exchange fluid changes state as it circulates in the loop. It evaporates when it absorbs heat dissipated by the equipment in the evaporator and rejects that heat when it condenses in one or more condensers situated at the level of the radiator. The fluid is circulated in the loop either passively, by capillary action, or using a mechanical pump upstream of the evaporator. The vapor and liquid phases are separated, except in the condenser and the evaporator, in which they flow in the same direction, unlike in a heat pipe, in which the two phases circulate in opposite directions in the same tube. This type of system has a capillary structure only at the level of the evaporator.
To increase the capacity of new generation high-power satellites to reject heat by radiation into space, it proves necessary to use deployable radiators to increase the dissipation surface areas available on the satellite. Given the powers to be dissipated, the surfaces of the body of the satellite are insufficient. The principle of the deployable radiator is to increase the radiation surface areas available on the satellite when they are in the deployed position, but a fluid loop is also used, as described above, to bring the dissipating power of the network of heat pipes supporting the dissipating equipment to the radiating surfaces of the deployable radiator, whilst allowing deployment of the radiator before it begins to operate.
For high-power satellites necessitating the presence of deployable radiators, a number of architectures are feasible for collecting and transporting heat from the equipment to the deployable radiators. Either the fluid loop collects the heat directly at the level of the equipment and transports it to the radiating surfaces of the radiator or a network of heat pipes collects the heat at the level of equipment and transports it to exchange areas in which the fluid loops recover it and feed it to the radiator. The first solution is suitable for single-phase fluid loops and the second solution is suitable for all types of loops.
In the case of the second solution, the network of heat pipes includes a primary network of heat pipes which collect and distribute the power to be dissipated from the equipment in a preferential direction. The first heat pipes are either integrated into the panel that supports them or mounted on the panel. A second network of heat pipes, known as coupling or crossing heat pipes, couples the heat pipes of the primary network together in a transverse direction. The first heat pipes are either integrated into the panel that supports them or mounted on the panel. The fluid loop thermally couples the radiating surface of the deployable radiator on which the condenser of the fluid loop is installed to the primary and coupling networks which drain heat from the equipment. As explained above, the fluid loop includes an evaporator for collecting the power to be dissipated from the networks of heat pipes on the panels of the satellite, a vapor line for feeding the power to be dissipated to a condenser, and a liquid return line for feeding the liquid back to the evaporator. A tank upstream of the evaporator stores liquid not circulating in the loop. The condenser is connected to the radiator, whose thermo-optical properties are adapted to reject power to the external medium. The vapor and liquid lines can be flexible to allow deployment of the radiator.
The efficiency of the deployable radiator (its rejection capacity) is largely dependent on the effective thermal gradient along the path between the dissipating equipment and the radiating surface. To a first approximation, the lower the thermal gradient, the more efficient the radiator. Also, the shorter the heat path, the lower the gradient. In particular, the operating temperature of the evaporator is a key factor in the efficiency of the loop.
Because heat is transferred primarily by conduction from the equipment to the radiating surface of the radiator, apart from the two-phase transfer in the heat pipes, the contact and exchange areas and surfaces must be thoroughly minimized in terms of their number, maximized in terms of their surface area, and optimized in terms of their quality of thermal contact.
Thus an object of the invention is to propose a heat transfer system making optimum use of radiators, in particular deployable radiators, by increasing the thermal rejection capacity of the satellite and improving the efficiency of the heat path by increasing the contact surface areas at nodes of the heat path.